Nova: A JavaScript and WebAssembly engine written in Rust

But: the lesser but still impactful performance benefit of ECS is the usage of Struct-of-Array vectors for data storage. JavaScript can still ruin that benefit by always accessing all parts and features of an Object every time it touches one, but it is a less likely thing to happen. So, there is a benefit that JavaScript code itself can enjoy.

Finally, there is one single "true System" in a JavaScript engine's ECS: the garbage collector. The GC will run through a good part of the engine heap, and you can fairly easily write it to be a batched operation where eg. "all newly found ordinary Objects" are iterated through in memory access order, have their mark checked, and then gather up their referents if they were unmarked. Rinse and repeat to find all live/reachable objects by constantly iterating mostly sequential memory in batches. This can also be parallelised, though then the batch queue needs to become shareable across threads.

The sweep of the heap after this is then a True-True System where all items are iterated in order, unmarked ones are ignored, marked ones are copied to their post-compaction location, and any references they hold are shifted down to account for the locations of items changing post-compaction.

So the project started as a "let's write a better JS engine than V8 in Rust!" kind of joke. Beyond that, I personally wanted to write in Rust, and it turns out that with sufficient abuse[1], the Rust borrow checker can be used to perform GC safety checks at compile time. This means that Nova can avoid rooting a lot of values during runtime because at compile time the borrow checker has already ensured that those values will never be used after a GC safepoint.

This is something that would not be feasible in other languages without significant custom checking infrastructure. As an example, Firefox / SpiderMonkey has a custom linter that checks that "no-GC" functions cannot accidentally call back into "GC" functions, but it is roughly a full-program analysis task and the checker is hand-written, custom code that sometimes lacks special cases for this function or that. In Nova, a "GC" function takes a move-only "GcScope" parameter by value (and all GC-able Values observe that value; when the value gets moved to a child call, all those observing Values invalidate; this is that GC safety checking), and "no-GC" functions take a copy "NoGcScope" parameter that can be created from a "GcScope" and that again observes the "GcScope". Through these, the borrow checker becomes the checker for this logic.

[1]: https://fosdem.org/2025/schedule/event/fosdem-2025-4394-abus...